The present invention relates to an analysis circuit for an amplitude-modulated electric signal comprising a frequency transposer.
The invention has numerous applications, particularly in the measurement of the distribution of the current along an antenna by the so-called modulated return radiation method, in the measurement of the radiated pattern or transmission diagram of an antenna and in more general terms, in ultra-high frequency measurements (analyser of networks, impedances, etc).
In order to give the principles of the invention, use will be made of the example of measuring the distribution of the current along an antenna by the modulated return radiation method, without this example being in any way limitative for the scope of the invention. The attached FIG. 1 illustrates this method.
An antenna 1 is supplied by a generator 2 across a circulator 3 (or hybrid T or coupler), generator 2 operates at a frequency fo. A small probe 4, formed from a conductive coil closed on a photoresistor, is moved along the antenna. The photoresistor of the probe is illuminated by light radiation guided by an optical fibre 5 supplied by a light-emitting diode 6. The latter is controlled by a circuit 7 comprising an oscillator operating at frequency fm. The amplitude-moldulated light radiation at frequency fm causes a variation in the resistance of the photoresistor inserted in the probe. The probe interacts with the antenna, in such a way that the electric signal reflected by the antenna is also amplitude modulated at frequency fm.
Thus, the signal supplied by circuit 3 is in form U(1+mcosw.sub.m t)cos(w.sub.o t+.phi.), in which U is an amplitude, which can be shown to be proportional to the square of the current flowing in the antenna at the location of the probe, in which w.sub.o is the pulsation 2.pi.fo, is a phase in which mcosw.sub.m t characterises the amplitude modulation at pulsation w.sub.m =2.pi.fm. This expression is valid, to within an interferring voltage due to leaks and mismatching of the system and which will be ignored hereinafter.
The spectrum of such a signal is formed by a carrier at frequency fo and two side bands at frequencies fo-fm and fo+fm. Such a signal carries amplitude and phase informations relative to the current flowing in the antenna to the right of the probe.
Such a method is e.g. described in the article by K. Itzuka entitled "photoconductive probe for measuring electromagnetic fields", published in IEE proceedings, vol 110, No. 10, October 1963.
The invention relates to a circuit able to analyze such a signal constituted by a carrier and two side bands. It carries the reference 100 in FIG. 1 and comprises a signal input 102 and an auxiliary input 104 for receiving part of the signal at frequency fm used for modulating the diode. The circuit according to the invention gives the amplitude and phase of the current circulating in the antenna and more generally of the high or ultra-high frequency signal which is to be analysed.
In order to process such a signal, the prior art has generally used a synchronous detection method. However, this method suffers from disadvantages, particularly with respect to the difficulty of obtaining a stable local isolator and providing a phase locked loop.